1. Field of the Invention
The present invention relates to a method of and apparatus for inspecting the width of a wiring line formed on a printed board.
2. Description of Prior Arts
A printed circuit board is provided with wiring patterns having lands and through holes. With reduction in size and weight as well as improvement in performance of electronic components, wiring patterns on a printed board are also refined and highly densified. Then, it is required to reduce the width of wiring lines as well as the diameter of through holes.
As to such narrow lines, inspection and management of widths thereof are more important than conventional lines having relatively wide width.
In inspection and management of line widths of the wiring pattern, a certain allowable range is set for the line widths. When the width of a line is within the range, the line is regarded as nondefective one. On the other hand, when the width of a line is out of the range, the line is regarded as defective.
Techniques of automatically inspecting the width of wiring lines have been developed, in which a pixel operator is employed as disclosed in Japanese Patent Laying-Open Gazette No. 59-74627 (1984), for example.
FIGS. 1(a) to 1(d) show a cross operator OP which is employed as an exemplary pixel operator.
FIG. 1(a) shows such a case that a defect MIS takes place in a wiring line L. Arms L.sub.1 and L.sub.2 of the operator OP on the line L and are parallel to the line L. The width of the line L is measured with arms L.sub.3 and L.sub.4 which are perpendicular to the line L. The line width is below a prescribed value at the defect MIS portion, whereby the defect MIS is detected.
FIG. 2 illustrates the cross operator OP in more detail. It is assumed that a logical level of "1" is obtained on the pixels of the operator OP located on a pattern P such as the line L, while the other logical level "0" is obtained on other pixels. Then, it is decided that the center O of the operator OP is on the line L if there are two arms L.sub.1 and L.sub.2 all pixels of which are "1" and the center O of the operator OP is "1". The other two arms L.sub.3 and L.sub.4 perpendicular to the two arms L.sub.1 and L.sub.2 provides continuous four pixels having the logical level "1", and the length corresponding to the four pixels is regarded as the line width W. When the line width W is below a prescribed value "5", for example, it is decided that there is the defect MIS on the pattern P.
The conventional method has the following two disadvantages.
(1) If the operator OP is applied to a land R having a pinhole PH as shown in FIG. 1(b), however, the pinhole PH is also inevitably decided as a defect MIS of the line L. Similarly, a mix pattern for power supply shown in FIG. 1(c) and a logo pattern shown in FIG. 1(d) are also inevitably decided as defects MIS of the lines L. Namely, the operator OP often mistakes nondefective patterns for defective lines, to cause false information and remarkably reduce reliability of the inspection.
(2) A region Aq (FIG. 4) in the vicinity of a corner of the line L cannot be effectively inspected in the conventional method, so that the region Aq is a "blind region" in the inspection. That is, if the operator OP scans a longitudinal line L from the lower position to the upper portion of the line L and the center O thereof reaches the blind region Aq, the arm L.sub.1 extends beyond the line L, so that part of pixels on the arm L.sub.1 provide the logical level of "0". Therefore, the blind region Aq is not detected as the line L although the same is essentially present on the line L.
This blind region Aq is in the form of a square, one edge of which is equal to the length of each arm of the operator OP. Hence, if the line L is relatively wide as shown in FIG. 4, it is possible to detect the defect MIS since the defect MIS located in the vicinity of the corner belongs to a detectable region Ap. On the other hand, when the line L is narrow as shown in FIG. 5, the defect MIS cannot be detected since the same is in the blind region Aq.
Respective arms of the operator OP may be reduced in size in order to reduce the blind region Aq. However, in this case, it is inevitable that a land R (FIG. 3) is also recognized as the line L and this portion is decided as defective.
A curved wide line pattern as shown in FIG. 6 can be decided as the line. However, a curved narrow line pattern as shown in FIG. 7 is not decided as the line even if the same has the same curvature as that of the case shown in FIG. 6, since there are no arms all pixels of which are "1".